1. Field of the Invention
The present invention relates to a scanning optical device used in a laser printer or the like.
2. Related Background Art
FIG. 1 shows a prior art scanning optical device of a laser printer. A collimator lens 3 and a rotating polygon mirror 4 are arranged on an optical axis of a laser diode 2 which emits light in accordance with a signal from a control circuit 1. The rotating polygon mirror 4 is rotated at a constant velocity in a direction of an arrow by a scanner motor 5. An f.theta. lens 6 is arranged on a reflection side of the rotating polygon mirror 4, and a photoconductor drum 7 which serves as a focal plane is arranged in a direction of transmission. A stationary mirror 8 is arranged in the vicinity of an end of the photoconductor drum 7. A condenser lens 9, a slit 10 and a photo-sensor (or light receiving element) 11 are arranged in sequence on the reflection side of the stationary mirror 8, and an output of the photo-sensor 11 is supplied to the control circuit 1.
The light emitted from the laser diode 2 is collimated by the collimator lens 3, reflected by the rotating polygon mirror 4, has the scanning velocity thereof on the photoconductor drum 7 fixed by the f.theta. lens, and is focused onto the photoconductor drum 7. When the scanning light is reflected by the stationary mirror in the vicinity of the end of the photoconductor drum 7, it is condensed by the condenser lens 9 and directed to the photo-sensor 11 through the slit 10. The output of the photo-sensor 11 is applied to the control circuit 1 as a horizontal synchronous signal. Image information to be recorded on the photoconductor drum 7 is supplied to the laser diode 2 a predetermined time after the input of the horizontal synchronous signal.
However, in the above prior art device, at least three lenses, the collimator lens 3, the f.theta. lens 6 and the condenser lens 9 are needed and they require high precision relative positioning.